Thermal properties, phase transitions, vibrational and reorientational dynamics of [Mn(NH3)6](NO3)2

Abstract

[Mn(NH3)6](NO3)2 crystallizes in the cubic, fluorite (C1) type crystal lattice structure (Fm\( \overline{3} \)m) with a = 11.0056 A and Z = 4. Two phase transitions of the first-order type were detected. The first registered on DSC curves as a large anomaly at TC1h = 207.8 K and TC1c = 207.2 K, and the second registered as a smaller anomaly at TC2h = 184.4 K and TC2c = 160.8 K (where the upper indexes h and c denote heating and cooling of the sample, respectively). The temperature dependence of the full width at half maximum of the band associated with the δs(HNH)F1u mode suggests that the NH3 ligands in the high temperature and intermediate phase reorientate quickly with correlation times in the order of several picoseconds and with activation energy of 9.9 kJ mol−1. In the phase transition at TC2c probably only a some of the NH3 ligands stop their reorientation, while the remainders continue to reorientate quickly with activation energy of 7.7 kJ mol−1. Thermal decomposition of the investigated compound starts at 305 K and continues up to 525 K in four main stages (I–IV). In stage I, 2/6 of all NH3 ligands were seceded. Stages II and III are connected with an abruption of the next 2/6 and 1/6 of total NH3, respectively, and [Mn(NH3)](NO3)2 is formed. The last molecule of NH3 per formula unit is freed at stage IV together with the simultaneous thermal decomposition of the resulting Mn(NO3)2 leading to the formation of gaseous products (O2, H2O, N2 and nitrogen oxides) and solid MnO2.